The invention relates to a process, method and a device for controlling the brake system of a motor vehicle.
Various patent document, such as German Documents DE 199 50 034 A1, DE 196 11 359 C1, DE 199 50 162 B4, DE 198 48 448 C2, DE 101 51 846 B4, DE 199 41 482 A1 and others, describe processes and devices which are supposed to serve to prevent vehicles from rolling away, in particular, but not necessarily, on slopes. In these, in at least one operational state, braking force on at least one wheel is usually held or actively increased with the actuated brake pedal or another actuation element, depending on or independently from the extent of actuation of the brake pedal or other actuation element. For the activation or deactivation of the function, different entry and/or triggering conditions together or alternatively are provided therein.
These processes or devices are known under various names, such as Start Driving Assistant, Mountain Assistant, Hill Hold(er), or Automatic Hold. For example, a hydraulic, mechanical, or pneumatic, electronically controllable braking system, using which the holding in place and preventing from rolling of the vehicle is achieved, is common to all these processes. Also, processes are known in which the rolling of the vehicle is prevented via the transmission (in particular automatic transmission). In all these functions an activation of the function is carried out, for example, by actuation of the brake pedal or the parking brake or via switching elements associated therewith or also other actuation elements (for example, switches, keys, or the like). A deactivation is done via the same or also other auxiliary elements (such as, for example, gas pedal position, clutch signal, drive torque signals of the motor, or the like or also after the expiration of a definite holding time). Important additional necessary auxiliary elements, control signals, switching elements, and the like for the representation of the functions are, for example, at least one speed signal for detecting that the vehicle is at a standstill, mechanical, pneumatic, or hydraulic actuators in the wheel brakes or in the feed lines to the wheel brakes, via which the braking action can be increased or reduced or held, at least temporarily (for example, control/regulation valves integrated in the form of a control or regulation device which is present, for example, ABS system, ASC/ASR system, DSC/ESP system), in given cases also longitudinal acceleration or slope sensors, by using which the longitudinal slope of the roadway, and from that the braking force required for secure standstill of the vehicle and the torque needed to set the vehicle in motion given the slope of the roadway, can be determined, and pressure sensors with whose aid the brake pressure or the braking torque applied to the wheel brake(s) can be determined.
Furthermore, from German Patent Document DE 103 22 125 A1 a device for controlling the brake system of a vehicle is known, in which device, while the vehicle is at a standstill, the braking force on at least one wheel of the vehicle is held depending on or independently of the extent of brake pedal actuation. Along with this, measures for detecting slippage of the vehicle or any danger of slippage when the device is activated are presented. This detection of slippage consists of a rotary or translational motion of the vehicle being determined (for example, via angular velocity/yaw rate/yaw speed or transverse acceleration) and, depending on the value determined, termination of the function occurs, even before the expiration of any predefined holding time, if the value permits the inference of slippage of the vehicle.
The present invention, provides processes of the type stated in the introduction, which improve the detection of slippage.
According to the invention, a process and method for detecting slippage is described, which operates in case of a low coefficient of friction, even during a holding process by which any rolling away of the vehicle is to be prevented first of all by driver-independent setting of a predefined braking force on preferably all the wheels, so that the braking force on at least one wheel which is not driven is released, at least temporarily, so that the wheel can roll freely. If this wheel does not roll it is detected that the holding power of the remaining wheels is sufficient to hold the vehicle fixed on the corresponding slope. If this wheel does roll it is detected that the vehicle is slipping via the remaining braked wheels. If a low coefficient of friction or slippage of the vehicle is detected in this way, the braking force is preferably released on all the wheels in order to be able to better maneuver the vehicle in this situation.
This process was originally developed for application in vehicles driven with a single axle. In vehicles with permanent all-wheel drive or all-wheel drive which can be switched to via certain actuation elements or automatically (electronically controlled), this process cannot be applied without further effort since one cannot assume that at a standstill the drive train is separated in such a manner that at least one wheel is not driven and is thus freely rolling.
In vehicles with permanent all-wheel drive and in vehicles which can be switched manually to all-wheel drive, carrying out the detection of slippage according to the process stated above is not possible or not reasonable in the switched-to state (firmly coupled).
In vehicles with at least one electronically controllable coupling unit for switching to and out of all-wheel drive it is to be assumed that the coupling units which produce the drive of the wheels are closed when the vehicle is at a standstill for reasons of traction. Coupling units are understood to mean, for example, longitudinal clutches or longitudinal locks and/or transverse differential clutches or transverse differential locks.
According to exemplary embodiments of the invention, it is ensured that in vehicles with at least one electronically controllable coupling unit for the production of an all-wheel drive, such as, for example, with an electronically controllable longitudinal clutch there is, for carrying out a detection of slippage, at least one wheel which is not driven. In particular, for achieving this goal it is proposed to open at least one coupling which is present.
This exemplary embodiment can, for example, include a switchable longitudinal lock or a controllable longitudinal clutch through which, for example, the front axle is decoupled from the drive. Should a controllable transverse differential lock also be present on at least one axle, the opening of this transverse differential lock and a reduction of pressure on one wheel of this axle may be sufficient.
In connection with this embodiment, it is first checked whether the coupling unit(s) required for carrying out the detection of slippage is(are) opened. If this is not the case, the required coupling unit(s) is(are) opened for the purpose of the detection of slippage.
If it is not possible to open the coupling units required to carry out the detection of slippage, then carrying out the detection of slippage may be suppressed. From this action, on the one hand, an improvement in convenience or avoidance of noise follows and, on the other hand, an improvement in wear behavior due to the fact that the system components needed for detecting slippage are not actuated. A useless actuation of components for the distribution of brake pressure is avoided in this example, which would otherwise be necessary for maintaining the holding energy fed into the system (in particular startup of the hydraulic pump in hydraulic brake control systems, actuating the valves or similar components in other brake control systems).
However, with the coupling unit opened while a test to detect slippage (slippage detection routine) is being carried out, the driver cannot set the vehicle in motion as rapidly as possible with optimal propulsion, as desired in given cases. It is therefore ensured in an extension according to the invention that the opened coupling unit(s) is(are) closed once again at the necessary time, in particular if there is a wish to set the vehicle in motion.
The coupling unit(s) which are decoupled due to a test for slippage should therefore not only be coupled in once again when a brake pressure reduction condition, e.g., sufficient drive torque of the motor, is present, that is, only at the time of driving away. In situations where the vehicle is set in motion rapidly this can lead to loss of performance or limitations on convenience.
According to exemplary embodiments of the invention, a gain in performance will therefore be achieved by prompt detection of a situation in which it is the desire of the driver to set the vehicle in motion rapidly and in a manner optimized for propulsion and by immediate (before the actual process of setting the vehicle in motion) restoration of a completely closed (coupled) all-wheel drive train. Closing the coupling unit(s) before the process of setting the vehicle in motion also increases comfort. Thereby it is ensured that in the time after the process of setting the vehicle in motion no discontinuity caused by closing of the coupling unit(s) arises in the motion of the vehicle, where such a discontinuity could be perceived by the occupants of the vehicle as a jerk. Also the service lifetime of the coupling unit(s) is extended by the closing of the coupling unit(s) in the (nearly) unloaded state.
The detection of this situation, in which setting the vehicle in motion rapidly and in a manner optimized for propulsion is desired by the driver, can be coupled to various signals which permit the desire of the driver to be recognized, for example,                pedal value (acceleration pedal actuated /not actuated) or corresponding value of another actuation unit. In this example, an alternative includes an acceleration pedal switch signal which senses an acceleration pedal position which corresponds to an acceleration pedal being depressed to between, for example, about 50% and about 100%        pedal path/gradient: overshooting of a certain threshold of only one quantity or a combination of both        clutch switch: in this example, an alternative includes a clutch switch signal which senses a clutch pedal position which corresponds to a clutch being closed to between 0 and, for example, about 50% (clutch switch sensing 100%, meaning that the clutch is completely closed would not be applicable)        clutch path/gradient: overshooting of a certain threshold of only one quantity or a combination of both        rotary speed/gradient of the motor: overshooting of a certain threshold of only one quantity or a combination of both        torque/gradient of the motor: overshooting of a certain threshold of only one quantity or a combination of both        (input/output) rotary speed/gradient of the transmission: overshooting of a certain threshold of only one quantity or a combination of both        (input/output) torque/gradient of the transmission: overshooting of a certain threshold of only one quantity or a combination of both        rotary speed (difference)/gradient for the coupling unit(s): overshooting of a certain threshold of only one quantity or a combination of both        torque (difference)/gradient for the coupling unit(s): overshooting of a certain threshold of only one quantity or a combination of both        
In this exemplary embodiment, the signals indicating pedal value, pedal path/gradient, clutch path/gradient, and so on reflect the direct wish of the driver. The other signals should be supported by additional signals (for example, pedal value (acceleration pedal actuated/not actuated) or pedal path/gradient) or can be drawn upon for support (clutch switch/signal 0−x %).
With an overshoot of a currently defined threshold, it can be assumed according to this exemplary embodiment that the driver desires a rapid process for setting the vehicle in motion. Thus, according to the invention, a control command which permits and/or restores the completely closed state of the drive train may be supplied in these situations.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.